In a hip hemiarthroplasty procedure, the proximal portion of a femur is replaced with a suitable prosthetic hip joint implant or implant assembly that articulatably mates directly with a patient's natural acetabulum. Two types of femoral prostheses are typically suitable for hip hemiarthroplasty procedures. One type is a bipolar prosthesis. In general, a bipolar hip prosthesis, such as disclosed in U.S. Pat. Nos. 3,813,699 and 3,863,273, includes a shell having an external surface which articulates with a patient's acetabulum and an internal surface which articulates with the spherical head member of a prosthetic femoral component. The other type of prosthesis is often referred to as a unipolar endoprosthesis in which the prosthetic femoral component includes a spherical head member which is large enough to articulate directly with the acetabulum.
One drawback to the successful use of bipolar hip prostheses is that they may become displaced after insertion. In particular, the spherical head of the femoral component may become dissociated from the shell. Such a dissociation may occur as the result of the abnormal twisting of a leg, or after a trauma such as a fall, such as might cause a dislocation in a natural hip joint. A problem with some prior art bipolar hip prostheses is that, in these situations, the dissociation can only be cured by further surgery.
U.S. Pat. No. 4,798,610 describes a bipolar femoral hip prosthesis having a floating locking ring that attempts to provide improvements over U.S. Pat. No. 4,241,463. According to its disclosure, the spherical head of this femoral prosthesis does not dislocate from the shell. Rather, in response to forces that would dislocate a natural hip, the shell disengages from the acetabulum. The dislocation may then be corrected in a manner similar to the manner in which the dislocation of a natural hip is corrected.
Dissociation of the components within the acetabulum is not necessarily preferred however. When correcting such a dissociation, it can be difficult to properly align the shell during correction. Also, dissociation of the components within the acetabulum and correction of such a condition can result in damage to the patient's natural bone. In addition, proper orientation of a floating locking ring, such as those described in U.S. Pat. Nos. 4,241,463 and 4,798,610, can be problematic, even during normal use of the prosthesis.